In rolling out the first draft of the Tax Cuts and Jobs Act, the biggest attempt to overhaul the tax code since 1986, House Ways and Means Committee Chair Kevin Brady used a hypothetical middle-class family to illustrate how much the bill would pay off for everyday Americans.

“With this bill, a typical middle-income family of four, earning $59,000 (the median household income), will receive a $1,182 tax cut,” a press release reads. Brady elaborated in a statement made while introducing the bill: “The Tax Cuts and Jobs Act will deliver real tax relief to Americans across the country — especially low- and middle-income Americans who have been struggling for far too long to earn a raise and get ahead.”

That’s definitely true in the short term, David Kamin, a law professor at New York University and former special assistant for economic policy to then-President Barack Obama, said in a phone interview. But unlike the tax cuts for richer Americans, within the next 10 years that hypothetical middle-class family will potentially be paying nearly $500 more than they would be if the tax code stayed the same. You can read the full analysis here.

So how does that work? The middle-class tax cut disappears for three major reasons, he explained.

“One reason it changes over time is that the bill includes a Family Flexibility Credit of $300 per person, so a total of $600 for a married couple,” Kamin said in a phone interview. “That expires starting in 2023.”

Over time, the middle class tax cut would get smaller, according to Kamin’s calculations. Mic/David Kamin

But there are two other reasons why the middle class tax bill would diminish over time, and both have to do with inflation — which is the amount that the price of goods and services goes up over time. The first is that overall, the Tax Cuts and Jobs Act would use a different, slower measure of inflation to calculate benefits than it does now.

But the bill also packs a one-two punch by doing away with personal exceptions, a benefit that’s indexed to inflation — and replaces it with two credits that are not. According to Kamin, that means the middle-class family Brady described is bound to receive a little bit less from the plan each year over time as prices go up.

“You can reasonably argue about what measure of inflation is best,” he said. “What you can’t argue is that if you flip over to a slower measure, many middle class families will get a slowly creeping up tax burden over time relative to what it would be under current law.”

Kamin said there’s no way to know for sure exactly why the bill is structured this way, but suspects it’s because there’s only so much money that can go around, and the middle class just isn’t a priority. The estate tax, for example, only applies to the wealthiest 0.2% of estates each year. Unlike the child tax credits, the reductions to the estate tax get bigger each year, before being phased out entirely after six years.

“This is a significant benefit to a very, very small group of people,” he said. “And a bill that gives very, very little to middle- and low-income Americans, which means they will end up losing from this over time.”